One of the primary methods for well stimulation in the production of hydrocarbons is hydraulic fracturing. Hydraulic fracturing is a method of using pump rate and hydraulic pressure to fracture or crack a subterranean formation. Once the crack or cracks are made, high permeability proppant, relative to the formation permeability, is pumped into the fracture to prop open the crack. When the applied pump rates and pressures are reduced or removed from the formation, the crack or fracture cannot close or heal completely because the high permeability proppant keeps the crack open. The propped crack or fracture provides a high permeability path connecting the producing wellbore to a larger formation area to enhance the production of hydrocarbons.
The development of suitable fracturing fluids is a complex art because the fluids must simultaneously meet a number of conditions. For example, they must be stable at high temperatures and/or high pump rates and shear rates that can cause the fluids to degrade and prematurely settle out the proppant before the fracturing operation is complete. Various fluids have been developed, but most commercially used fracturing fluids are aqueous based liquids that have either been gelled or foamed. When the fluids are gelled, typically a polymeric gelling agent, such as a solvatable polysaccharide, for example guar and derivatized guar polysaccharides, is used. The thickened or gelled fluid helps keep the proppants within the fluid. Gelling can be accomplished or improved by the use of crosslinking agents or crosslinkers that promote crosslinking of the polymers together, thereby increasing the viscosity of the fluid. One of the more common crosslinked polymeric fluids is borate crosslinked guar.
The recovery of fracturing fluids may be accomplished by reducing the viscosity of the fluid to a low value so that it may flow naturally from the formation under the influence of formation fluids. Crosslinked gels generally require viscosity breakers to be injected to reduce the viscosity or “break” the gel. Enzymes, oxidizers, and acids are known polymer viscosity breakers. Enzymes are effective within a pH range, typically a 2.0 to 10.0 range, with increasing activity as the pH is lowered towards neutral from a pH of 10.0. Most conventional borate crosslinked fracturing fluids and breakers are designed from a fixed high crosslinked fluid pH value at ambient temperature and/or reservoir temperature. Optimizing the pH for a borate crosslinked gel is important to achieve proper crosslink stability and controlled enzyme breaker activity.
While polymers have been used in the past as gelling agents in fracturing fluids to carry or suspend solid particles as noted, such polymers require separate breaker compositions to be injected to reduce the viscosity. Further, such polymers tend to leave a coating on the proppant and a filter cake of dehydrated polymer on the fracture face even after the gelled fluid is broken. The coating and/or the filter cake may interfere with the functioning of the proppant. Studies have also shown that “fish-eyes” and/or “microgels” present in some polymer gelled carrier fluids will plug pore throats, leading to impaired leakoff and causing formation damage.
Recently it has been discovered that aqueous drilling and treating fluids may be gelled or have their viscosity increased by the use of non-polymeric viscoelastic surfactants (VES). These VES materials are advantageous over the use of polymer gelling agents, since they are low molecular weight surfactants, in that they are less damaging to the formation, without a fluid-loss additive present leave no filter cake on the formation face, leave very little coating on the proppant, and do not create microgels or “fish-eyes”. Progress has also been made toward developing internal breaker systems for the non-polymeric VES-based gelled fluids, that is, breaker systems that use products that are incorporated and solubilized within the VES-gelled fluid that are activated by downhole conditions that will allow a controlled rate of gel viscosity reduction over a rather short period of time of 1 to 4 hours or so, similar to gel break times common for conventional crosslinked polymeric fluid systems.
Furthermore, although VES-gelled fluids are an improvement over polymer-gelled fluids from the perspective of being easier to clean up the residual gel materials after the fluid viscosity is broken and the fluid produced or flowed back, improvements need to be made in cleaning-up from operations employing VES-gelled fluids.
It would be desirable if clean-up methods could be devised to more completely and easily remove well completion fluids gelled with and composed of viscoelastic surfactants, particularly the remnants or deposits left by such fluids.